Impulse SIO-104+2 (3502) - Manual (Page 6)

Card Setup
Sealevel Systems SIO-104+2 Page 4
Switches SW1B and SW2B
RS-485 Enable Modes (SW1B and SW2B position 1)
RS-485 is ideal for multi-drop or network environments. RS-485 requires a tri-state driver that will allow the
electrical presence of the driver to be removed from the line. The driver is in a tri-state or high impedance condition
when this occurs. Only one driver may be active at a time and the other driver(s) must be tri-stated. The output
modem control signal Request To Send (RTS) is typically used to control the state of the driver. Some
communication software packages refer to RS-485 as RTS enable or RTS block mode transfer.
One of the unique features of the SIO-104+2 is the ability to be RS-485 compatible without the need for special
software or drivers. This ability is especially useful in Windows, Windows NT, and OS/2 environments where the
lower level I/O control is abstracted from the application program. This ability means that the user can effectively
use the SIO-104+2 in an RS-485 application with existing (i.e. standard RS-232) software drivers.
DIP-Switch SW2A and SW2B are used to control the RS-485 mode functions for the driver circuit. The selections
are:
‘Auto’ enable (Switch position 1 ‘Off’). The ‘Auto’ enable feature automatically enables/disables the RS-485
interface via on board circuitry.
‘RTS’ enable (Switch position 1 ‘On’) The ‘RTS’ mode uses the ‘RTS’ modem control signal to enable the RS-485
interface and provides backward compatibility with existing software products.
Electrical Interface Selection (Positions 2 and 3)
Positions 2 and 3 select the Electrical Interface for the SIO-104+2, the following table represents the only valid
options:
Mode M0 M1
RS232 OFF OFF
RS422 OFF ON
RS485 with echo ON OFF
RS485 no echo ON ON